The pgrowlocks module provides a function to show row
locking information for a specified table.
By default use is restricted to superusers, roles with privileges of the
pg_stat_scan_tables role, and users with
SELECT permissions on the table.
pgrowlocks(text) returns setof record
The parameter is the name of a table. The result is a set of records, with one row for each locked row within the table. The output columns are shown in Table F.21.
Table F.21. pgrowlocks Output Columns
| Name | Type | Description |
|---|---|---|
locked_row | tid | Tuple ID (TID) of locked row |
locker | xid | Transaction ID of locker, or multixact ID if multitransaction; see Section 76.1 |
multi | boolean | True if locker is a multitransaction |
xids | xid[] | Transaction IDs of lockers (more than one if multitransaction) |
modes | text[] | Lock mode of lockers (more than one if multitransaction),
an array of Key Share, Share,
For No Key Update, No Key Update,
For Update, Update. |
pids | integer[] | Process IDs of locking backends (more than one if multitransaction) |
pgrowlocks takes AccessShareLock for the
target table and reads each row one by one to collect the row locking
information. This is not very speedy for a large table. Note that:
If an ACCESS EXCLUSIVE lock is taken on the table,
pgrowlocks will be blocked.
pgrowlocks is not guaranteed to produce a
self-consistent snapshot. It is possible that a new row lock is taken,
or an old lock is freed, during its execution.
pgrowlocks does not show the contents of locked
rows. If you want to take a look at the row contents at the same time, you
could do something like this:
SELECT * FROM accounts AS a, pgrowlocks('accounts') AS p
WHERE p.locked_row = a.ctid;
Be aware however that such a query will be very inefficient.
=# SELECT * FROM pgrowlocks('t1');
locked_row | locker | multi | xids | modes | pids
------------+--------+-------+-------+----------------+--------
(0,1) | 609 | f | {609} | {"For Share"} | {3161}
(0,2) | 609 | f | {609} | {"For Share"} | {3161}
(0,3) | 607 | f | {607} | {"For Update"} | {3107}
(0,4) | 607 | f | {607} | {"For Update"} | {3107}
(4 rows)
Tatsuo Ishii